IR detector structure and method of making

ABSTRACT

An IR detector structure is made from a CdTe substrate by polishing the  C subtrate, and transferring the cleaned substrate to a chamber for successive epitaxial growth of HgCdTe and CdTe layers insitu without removal reducing contamination at the interfaces due to exposure to the atmosphere.

The Government has rights in this invention under Contract No.DAABO7-86-C-F069 with the Department of the Army.

This invention relates in general to an IR detector structure and to itsmethod of making and in particular, to such a detector that can be madeby molecular beam epitaxy (MBE) or metal oxide chemical vapor deposition(MOCVD) of HgCdTe.

BACKGROUND OF THE INVENTION

A variety of IR detector structures can be fabricated from HgCdTe. Oneof these detector types is the Metal-Insulator-Semiconductor (MIS). Withthe MIS detector, a voltage is applied to the insulated gate such thatmajority carriers are depleted from the area immediately under the gate.Photo-generated minority signal carriers are then collected in thedepleted region and read out. The gate level is typically metal and iffront-side illumination is desired the metal is kept very thin (<100angstroms). The very thin metal is difficult to deposit uniformlyespecially over stepped regions. Also the gate is much too thin to beused for self-aligned implants, if self-aligned implants prove to beuseful.

SUMMARY OF THE INVENTION

The general object of this invention is to provide a method of making anIR-detector structure. A more particular object of the invention is toprovide a method of making such a structure using HgCdTe photodetectorarrays and a transparent gate for the photodetector arrays. A stillfurther object of the invention is to provide such a method wherein thearrays of the detector structure can be front side illuminated andself-aligned implants are possible and in which the gate can be adjustedto act as a cold "cut-on" filter.

It has now been found that the aforementioned objects can be attained bydepositing a transparent gate for HgCdTe photodetector arrays insituduring MBE or MOCVD detector material growth.

More particularly a CdTe cap/insulator layer is grown insitu after MBEgrowth of HgCdTe detector layer(s). The CdTe acts as a high qualitydielectric layer since the CdTe growth is done insitu and the CdTe is agood insulator.

A layer of HgCdTe is then grown on the CdTe insulator layer describedabove. This layer can act as a conductor gate layer for MIS structures.The crystal quality of the layer is not critical since the layerfunctions primarily as a conductor. Also, the gate layer compositionwill have a relatively high cadmium telluride mole fraction (compared tothe detector layer) making the gate transparent at the detector'sabsorption wavelength. If desired, the composition of the gate materialcan be adjusted for the gate to act as a "cut-on" filter. The HgCdTetransparent gate layer can be preferentially etched over the CdTeinsulator using a dry etch to form the desired gate pattern. The gatelayer is also relatively dense and can be grown reasonably thick (a fewtenths of a micron) to act as a blocking layer for self-alignedimplants.

DESCRIPTION OF THE DRAWING

The drawing is a cross sectional view of a detector structure accordingto the invention.

Referring to the drawing, there is shown a CdTe substrate 10 of about 10mils in thickness upon which is epitaxially grown insitu a layer ofHgCdTe narrow gap or absorber layer, 12. This layer is the detectorlayer and is about 10 micrometers in thickness. Upon layer 12, there isepitaxially grown insitu an HgCdTe wide gap or signal storage layer, 14of about 0.1 micrometer to 1 micrometer in thickness. Upon layer 14,there is epitaxially grown insitu a CdTe insulator layer, 16 of about0.1 to 0.2 micrometer in thickness. Upon layer 16, there is epitaxiallygrown insitu a HgCdTe wide gap or gate layer, 18 of about 0.5 micrometerin thickness.

The structure allows for detector arrays to be front-side illuminated.Implants self-aligned to the transparent gate can also be implemented.Then too, the gate material can be deposited insitu during MBE or MOCVDdetector and insulator layer growth. The gate can also act as a cold"cut-on" filter for the LWIR or MWIR detectors. The number of postgrowth fabrication steps are also reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A CdTe or CdZnTe substrate is polished using one of several surfacepreparation techniques, for example a light surface etch in a dilutesolution of Bromine/Methanol. The substrate thickness is not criticalfor this device structure since the active detector area will beilluminated from the front (gate) side. The cleaned substrate is thentransferred to an MBE or MOCVD chamber for epitaxial growth of HgCdTeand CdTe layers. These epitaxial layers should be grown successively inthe MBE or MOCVD chamber without removal. This reduces contamination atthe interfaces due to exposure to the atmosphere.

MBE epitaxial growth is achieved by codeposition of Mercury, Telluriumand Cadmium Telluride onto the substrate which is heated to about 200°C. Deposition rate and composition are controlled by adjusting thetemperature of the effusion source cells for the Hg, Te and CdTe.Approximate temperatures would be 700° C. for CdTe, 400° C. for Te and200° C. for Hg. MOCVD epitaxial growth is achieved on substrates heatedto about 400°0 C. by chemical vapor deposition. Typical gas sources aredimethylcadmium for Cd and diethyltelluride for Te. Elemental mercury isprovided by evaporation. Several alternate gas sources are also usedwith, in general, different substrate temperatures.

Epitaxial growth of HgCdTe by MBE and MOCVD has been demonstrated onseveral crystal orientation substrates. The most popular orientationsbeing (100), (111) and (211). The first epitaxial layer to be grown is anarrow gap HgCdTe layer. A typical thickness for longwave IRapplications would be 10 μm. By narrow (band) gap is meant a layer whoseCadmium mole fraction (X) is adjusted to absorb infra-red radiation ofthe desired long wavelength. For example, a Cadmium mole fraction of0.22 would result in a cutoff wavelength of about 10 μm at 77K. Thevariable band gap properties of HgCdTe are well known in the industry.

Film carrier type by either epitaxial growth technique can be controlledby stoichiometry. For example, a mercury excess will result in an n-typefilm and a mercury deficiency will result in a p-type film. Carrier typecan also be controlled by extrinsic doping by any one of severaltechniques. The narrow gap and wide gap layers shown in the drawing mustbe of the same electrical type. The wide gap epitaxial layer is grownnext. By wide (band) gap is meant a Cadmium mole fraction which isgreater than the narrow gap absorber layer and will thus be transparentto radiation which is absorbed in the narrow gap layer. A typicalthickness of the wide gap layer would be 0.5 μm. This layer is thedetector depletion region in which photogenerated signal charge isstored.

A CdTe electrical insulating layer is grown next by suppressing the Teand Hg sources during MBE growth or the Te source during MOCVD growth.This CdTe layer forms the insulator of the MIS(metal-insulator-semiconductor) structure. By qrowing the CdTe layerepitaxially, insitu, a minimum of interface states will result.

The final layer of the MIS structure is the transparent HgCdTe gate. Theaddition of this layer is the basis of this invention. The layer isgrown epitaxially, insitu, in the MOCVD or MBE growth chamber. Thecrystal quality of this gate level is not critical since the layer actsonly as an electrically conducting region and is not used for signalcharge transport. The gate is transparent for frontside illuminationbecause the cadmium mole fraction is adjusted to allow transmission oflonger wavelength radiation which is then absorbed in the narrow gaplayer.

I wish it to be understood that I do not desire to be limited to theexact details of construction shown and described for obviousmodification will occur to a person skilled in the art.

What is claimed is:
 1. Method of making an IR detector structure from aCdTe substrate, said method including the steps of:(A) polishing theCdTe substrate (B) transferring the cleaned substrate to a chamber forsuccessive epitaxial growth of HgCdTe and CdTe layers insitu withoutremoval reducing contamination at the interfaces due to exposure to theatmosphere, (C) growing an epitaxial layer of a narrow gap HgCdTe ofabout 10 μm in thickness onto the CdTe substrate (D) growing anepitaxial layer of wide gap HgCdTe of about 0.5 μm in thickness onto thenarrow gap layer, (E) growing an electrical insulating epitaxial layerof CdTe of about 0.1 to 0.2 μm in thickness onto the wide gap layer ofHgCdTe, and (F) growing an epitaxial transparent HgCdTe gate layer ofabout 0.5 μm in thickness onto the CdTe layer.
 2. Method according toclaim 1 where in step (A) the CdTe substrate is polished with a lightsurface etch in a dilute solution of bromine/methanol.
 3. Methodaccording to claim 1 where in step (B) the chamber for epitaxial growthis selected from the group consisting of MBE and MOCVD.
 4. Methodaccording to claim 3 where in step (B) the chamber is MBE.
 5. Methodaccording to claim 3 wherein step (B) the chamber is MOCVD.
 6. Methodaccording to claim 4 wherein MBE epitaxial growth is achieved bycodeposition of mercury, tellurium and cadmium telluride onto thesubstrate which is heated to about 200° C. and wherein deposition rateand composition are controlled by adjusting the temperature of theeffusion source cells for the Hg, Te and CdTe, approximate temperaturebeing about 700° C. for CdTe, about 400° C. for Te, and about 200° C.for Hg.
 7. Method according to claim 5 wherein MOCVD epitaxial growth isachieved on substrates heated to about 400° C. by chemical vapordeposition, typical gas sources being demethylcadmium for Cd anddiethyltelluride for Te, elemental mercury being provided byevaporation.
 8. An IR detector structure, said structure comprising acadmium telluride substrate, an epitaxial layer of narrow gap HgCdTe onsaid substrate and wherein said epitaxial layer has been grown insitu, awide gap epitaxial layer of HgCdTe on said narrow gap layer and whereinsaid wide gap layer has been grown insitu, a CdTe electrical insulatinglayer epitaxially grown insitu on said wide gap layer, and a transparentHgCdTe gate layer grown epitaxially, insitu, on said CdTe layer.